1. Field of the Invention
This invention generally relates to integrated circuit (IC) fabrication and, more particularly, to forming a printed transistor with a short channel length that is sub-resolutional to the movement of the printing equipment.
2. Description of the Related Art
As noted in Wikipedia, printed electronics is a set of prig ting methods used to create electrical devices on various substrates. Printing typically uses common printing equipment or other low-cost equipment suitable for defining patterns on material, such as screen printing, flexography, gravure, offset lithography and inkjet. Electrically functional electronic or optical inks are deposited on the substrate, creating for example, active or passive devices, such as thin film transistors (TFTs) or resistors. These processes can utilize any liquid phase material, including, but not limited to, solutions, mixtures, and dispersions containing organic semiconductors, inorganic semiconductors, organic dielectrics, inorganic dielectrics, metallic conductors, oxide conductors, organic conductors, nanowires, nanoparticles, nanotubes, and nanotubes.
The attraction of printing technology for the fabrication of electronics mainly results from the possibility of preparing stacks of micro-structured layers (and thereby thin-film devices) over large areas in a much simpler and cost-effective way, as compared to conventional electronics. Also, the ability to implement new or improved functionalities (e.g. mechanical flexibility) plays a role.
Organic field-effect transistors and integrated circuits can be prepared completely by means of mass-printing methods. The selection of print methods for the different layers is determined by dimensional requirements and the properties of printed materials, as well as economic and technical considerations of the final printed products. Optimal resolution of these considerations typically results in a combination of several print methods for the fabrications of the devices, as opposed to a single method.
Inkjets are flexible and versatile, and can be set up with relatively little effort. Inkjets are currently the most commonly used method for the preparation of printed electronics. Inkjet printing is well suited for low-viscosity, soluble materials like organic semiconductors, and has proved useful in printing dispersed particles, like inorganic metal inks, with some observed difficulties due to nozzle clogging. Because ink is deposited via droplets, thickness and dispersion homogeneity is reduced. Inkjet printing is preferable for organic semiconductors in organic field-effect transistors (OFETs) and organic light-emitting diodes (OLEDs) due to viscosity constraints and high ink costs, but also OFETs completely prepared by this method have been demonstrated. Frontplanes and backplanes of OLED-displays, integrated circuits, organic photovoltaic cells (OPVCs), and other devices can be prepared with ink jets.
As mentioned above, both organic and inorganic materials are commonly used for printed electronics. These ink materials must be available in liquid form, for solution, dispersion, or suspension. Additionally, they have varying functionality, to serve as conductors, semiconductors, dielectrics, or insulators. Electronic functionality and printability can interfere with each other, mandating careful optimization. For example, a higher molecular weight in polymers enhances conductivity, but diminishes solubility. For printing, viscosity, surface tension, and solid content must be tightly controlled. Cross-layer interactions such as wetting, adhesion, and solubility as well as post-deposition drying procedures affect the outcome. Additives often used in conventional printing inks are unavailable, because they often defeat electronic functionality.
Metal inks are also commonly used in printed electronics for reasons of improved conductivity and potential for surface functionality, as compared to their organic counterparts. Silver, gold, and copper nanoparticle inks are used with all of the printing processes described above. Metal lines are commonly formed in printed electronic applications by inkjet printing of metal nanoparticle or metal precursor inks onto a substrate. The line width and line shape are dictated by the printed volume of ink and the interaction of the ink with the surface of the substrate. It is important to tailor the surface energy of the substrate for a specific ink to achieve the desired line characteristics. In order to create a thin film transistor (TFT) by inkjet printing, two inkjet printed metal layers are typically required: gate metal and source/drain metal. The source/drain print characteristics are of particular interest because they typically determine the transistor channel length (L).
The accuracy of inkjet drop placement limits how far down the channel length can be reasonably scaled, while preserving device yield. Misplaced drops can cause the printed source and drain lines to merge for example and cause an electrical short. There are many factors that affect drop placement. Some of these factors are hardware limitations of the printer. A couple examples of hardware limitations are the inherent accuracy of the substrate stage movement or movement of the inkjet cartridge. Typically, these positional limitations are on the order of 10 μm for commercially available printers today. The repeatable printing of lines with spacings below these values is unlikely without other process improvements.
TFT drain current is generally considered to be inversely proportional to the channel length of the device. So in order to improve the ON current and switching speed, the semiconductor performance must be enhanced (e.g., greater electron mobility) or the channel length reduced. This improvement becomes even more important in the case of display backplanes where the desire is to minimize the footprint of the backplane circuits. With a digital fabrication technique such as inkjet printing, the channel length of a printed device is determined by how close two adjacent electrode lines can be printed without the lines merging along their lengths. The drop placement and stage movement limitations of commercially available inkjet printers are on theoretically the order of 5 to 10 microns (μm), and practically, the inkjet printing of source/drain electrodes for TFTs with channel lengths below 30 μm is difficult to produce, and is not reproducible with current commercially available inkjet printers and ink formulations.
It would be advantageous if the above-mentioned printer hardware limitations could be circumvented with a process that reduced the spacing between printed features.